Since its discovery in 1928 (Diels, O.; Alder, K. Liebigs Ann. Chem. 1928, 460, 98-122) more than 17,000 papers have been published concerning the synthetic, mechanistic and theoretical aspects of the Diels-Alder reaction (Fringuelli, F.; Taticchi, A. The Diels-Alder Reaction 2002, John Wiley & Sons Ltd.). This [4+2]cycloaddition reaction in which a conjugated diene adds to a dienophile is probably the most widely used methodology in organic synthesis today.
Many conjugated dienes and dienophiles have been reported for use in the Diels-Alder reaction. Dienes will react as long as the two double bonds have, or can assume, a cisoid geometry. For this reason cyclic dienes are generally more reactive than the acyclic ones since their cisoid geometry is fixed. Dienophiles, which are much more numerous than dienes, are molecules possessing a double or triple bond.
The Diels-Alder reaction is reversible and the direction of cycloaddition is favoured because two π bonds are replaced by two σ bonds. The reverse reaction occurs if the diene and/or dienophile are particularly stable molecules or when one of them can be easily removed or consumed in a subsequent reaction. The retro Diels-Alder reaction usually requires high temperatures in order to overcome the high activation barrier.
The Diels-Alder reaction is used frequently in organic chemistry, but has also received some attention in polymer chemistry. Polymers have been synthesised via consecutive Diels-Alder reactions in various ways. Mikroyannidis et al. reported the Diels-Alder reaction between AA/BB monomers, where AA is a monomer having reactive diene groups and BB is a monomer having reactive dienophile groups (Mikroyannidis, J. A. J. Polym. Sci. Part A—Polym. Chem. 1992, 30, 2017-2024).
Kamahori et al. described the synthesis of alternate polymers via the Diels-Alder polymersation of difurfuryl terephthalate—a bisdiene (AA) with N,N′-hexamethylene bismaleimide—a bisdienophile (BB) (Kamahori, K; Tada, S.; Ito, K.; Itsuno, S. Macromolecules 1999, 32, 541-547). By choosing the right ratio of the two monomers, optically active polymers could be prepared. Finally, the Diels-Alder reaction has been used for AB monomers, where the monomer behaves both as a diene and a dienophile, e.g. a cyclopentadiene group (Stille, J. K.; Plummer, L. J. Org. Chem. 1961, 26, 4026-4029).
Another interesting application of this reaction in polymers is the formation of thermally reversible networks. Polymers bearing pendant diene or dienophile groups have been cross-linked by reaction with a bisdienophile or a bisdiene respectively. Goussé et al. reported the synthesis of styrene copolymers bearing pendant furan moieties and their Diels-Alder based modifications with mono or bismaleimide (Goussé, C.; Gandini, A.; Hodge, P. Macromolecules 1998, 31, 314-321). The reaction proceeded as expected and the original copolymers could be recovered after thermal treatment. Jones et al. reported the synthesis of poly(ethylene terephthalate-co-2,6-anthracenedicarboxylate), where the anthracene unit was then used as a diene for the Diels-Alder reaction with various bisdienophiles (Jones, J. R.; Liotta, C. L.; Collard, D. M.; Schiraldi, D. A. Macromolecules 1999, 32, 5786-5792). This crosslinking reaction was thermally reversible, but at the temperature required (250° C.), the copolymer was prone to thermal decomposition. The synthesis of poly(hexyl acrylate-2-furfuryl methacrylate) copolymers was reported by Gheneim et al. (Gheneim, R.; Perez-Berumen, C.; Gandini, A. Macromolecules 2002, 35, 7246-7253). These copolymers produced cross-linked elastomers in high yields on addition of bismaleimides in dichloromethane. The thermal reversibility of this reaction was also confirmed. Liu et al. (Liu, Y. L.; Hsieh, C. Y.; Chen, Y. W. Polymer 2006, 47, 2581-2586) prepared cross-linked polyamides and polyamide gels from maleimide having polyamides and a trifunctional furan compound. The crosslinking and gel formation in dimethyl acetamide was shown to be thermally reversible, and this reversibility could be adjusted by the content of maleimide groups in the polyamide. Similarly, thermally re-mendable cross-linked polymeric materials have been reported by Chen et al. (Chen, X.; Wudl, F.; Mal, A. K.; Shen, H.; Nutt, S. R. Macromolecules 2003, 36, 1802-1807 and Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Sheran, K.; Wudl, F. Science 2002, 295, 1698-1702).
All the above examples concern the Diels-Alder reaction between one modified copolymer having a diene or a dienophile, with a small molecule bisdienophile or bisdiene crosslinker. Chujo et al. reported the Diels-Alder reaction between two modified copolymers, one having a dienophile—maleimide modified poly(N-acetylethylenimine) and the other having a diene—furan modified poly(N-acetylethylenimine) (Chujo, Y.; Sada, K.; Saegusa, T. Macromolecules 1990, 23, 2636-2641). These two copolymers were dissolved in methanol and left under dark conditions at room temperature for 1 week, in which time a gel formed. The gel swelled in water to form a hydrogel, and the reversible interconversion between this polyoxazoline hydrogel and the linear soluble polymers by a change in temperature was described. Huglin's group published their work on the synthesis of poly(styrene-co-furfuryl methacrylate) copolymers and the subsequent Diels-Alder and retro Diels-Alder reactions of these polymers with 1,2′-(methylendi-4,1-phenylene)bismaleimide in chloroform (Goiti, E.; Huglin, M. B.; Rego, J. M. Polymer 2001, 42, 10187-10193; Goiti, E.; Huglin, M. B.; Rego, J. M. Macromol. Rapid Commun. 2003, 24, 692-696; Goiti, E.; Huglin, M. B.; Rego, J. M. Eur. Polym. J. 2004, 40, 219-226; and Goiti, E.; Huglin, M. B.; Rego, J. M. Eur. Polym. J. 2004, 40, 1451-1460). The forward reaction occurred over time at room temperature, then on heating to 77° C. and above, the retro D-A process took place (Goiti, E.; Huglin, M. B.; Rego, J. M. Macromol. Rapid Commun. 2003, 24, 692-696).
As can be seen from the literature examples given above, the furan ring is one of the most important heterocycles used as the diene in Diels-Alder reactions. Furfural is an industrial commodity obtained from a wide variety of agricultural residues, and the pronounced dienic nature of the furan ring makes is particularly suitable in terms of kinetics and yields (Gheneim, R.; Perez-Berumen, C.; Gandini, A. Macromolecules 2002, 35, 7246-7253). On the dienophile side, maleimides are among the most commonly used reagents because of their high reactivity. The electron-attracting substituents attached to the double bond in maleimides promote the Diels-Alder reaction with furan compounds (Gheneim, R.; Perez-Berumen, C.; Gandini, A. Macromolecules 2002, 35, 7246-7253). The Diels-Alder reaction between a difuran and a bis(maleimide) has been used to obtain imide resins (Bibiao, J.; Jianjun, H.' Wenyun, W.; Luxia, J.; Xinxian, C. Eur. Polym. J. 2001, 37, 463-470), optically active materials (Kamahori, K, Tada, S., Ito, K., Itsuno, S. Macromolecules 1999, 32, 541-547), and even to synthesis of dendrons based on the furan-maleimide adducts (McElhanon, J. R.; Wheeler, D. R. Org. Lett. 2001, 3, 2681-2683). However, all of the literature examples of using the Diels-Alder reaction in polymer chemistry are carried out in organic solvent. No studies have been reported of using aqueous media for the Diels-Alder crosslinking. In 1980 Rideout et al. showed that some Diels-Alder reactions were vastly accelerated (up to 700 times faster) when carried out in aqueous media (Rideout, D. C.; Breslow, R. J. Am. Chem. Soc. 1980, 102, 7816-7817). When reacting anthracene-9-carbinol with N-ethylmaleimide they found the reaction to be slower in polar solvents than in nonpolar hydrocarbon solution, with the exception of water where the rate was very fast. This acceleration was therefore put down to a hydrophobic effect—enforced hydrophobic interactions and hydrogen bond interactions—rather than a polarity effect.